A Dopant Replacement‐Driven Molten Salt Method toward the Synthesis of Sub‐5‐nm‐Sized Ultrathin Nanowires

The high‐temperature molten‐salt method is an important inorganic synthetic route to a wide variety of morphological phenotypes. However, its utility is limited by the fact that it is typically incapable of producing ultrathin (<5 nm diameter) nanowires, which have a crucial role in novel nanotechnology applications. Herein, a rapid molten salt‐based synthesis of sub‐5‐nm‐sized nanowires of hexagonal tungsten oxide (h‐WO 3 ) that is critically dependent on a substantial proportion of molybdenum (Mo) dopant is described. This dopant‐driven morphological transition in tungsten oxide (WO 3 ) may be attributable to the collapse of layered structure, followed by nanocluster aggregation, coalescence, and recrystallization to form ultrathin nanowires. Interestingly, due to the structural properties of h‐WO 3 , the thus‐formed ultrathin nanowires are demonstrated to be excellent photocatalysts for the production of ammonia (NH 3 ) from nitrogen (N 2 ) and water. The ultrathin nanowires exhibit a high photocatalytic NH 3 ‐production activity with a rate of 370 µmol g −1 h −1 and an apparent quantum efficiency of 0.84% at 420 nm, which is more than twice that obtained from the best‐performing Mo‐doped W 18 O 49 nanowire catalysts. It is envisaged that the dopant replacement‐driven synthetic protocol will allow for rapid access to a series of ultrathin nanostructures with intriguing properties and increase potential applications.